[RFC] Add platform checks to replace config.h.in hardcoded values

Basically cmake checks for some of the hardcoded defines in config.h.in

* Commented out a bunch of defines that are not being used (and neither are the functions they refer to)
* Not quite sure what to do about tgetent related checks, these should probably be handled when looking for curses/term
* Some of the more tricky checks are still missing, iconv(), st_blksize, langinfo
* In some cases I fail immediately for functions/headers that are implicitly assumed to be available (setenv(), lstat(), sys/wait.h)
@equalsraf I learned CMake a bit :-) In CMake source tree I found the module [```CheckVariableExists```](http://cmake.org/gitweb?p=cmake.git;a=blob_plain;f=Modules/CheckVariableExists.cmake;hb=HEAD). This ```check_c_source_compiles(...)``` can be replaced with ```check_variable_exists(_nl_msg_cat_cntr HAVE_NL_MSG_CAT_CNTR)```. At least on my machine it works as intended.